1. Field of the Invention
The present invention relates to an apparatus and method for controlling the supply of power to an induction motor, and in particular to an induction motor control apparatus and control method whereby an induction motor can be operated with a high degree of efficiency under varying conditions of motor load.
2. Description of the Related Art
The present invention is related to a pending U.S. patent application, Ser. No. 07/776,117 with filing date Oct. 15, 1991, by the assignees of the present invention.
In controlling an induction motor to operate with high efficiency over a wide range of values of motor load, the basic objective is to apply an appropriate value of drive voltage to the motor for the load that is being currently imposed. If for example an excessively high value of supply voltage (in relation to the required level of motor torque) is applied while the motor is driving a very light load, then excessive drive current will flow, so that the operating efficiency is low. If however the supply voltage is insufficiently high in such a case, then a sudden increase in the motor load may result in stalling of the motor, or to unstable operation. Various schemes have been proposed in the prior art for controlling the supply of power to an induction motor such as to maximize the operating efficiency. However in general such schemes are deficient with regard to preventing stalling or instability during low-load operation of an induction motor, or result in excessive power consumption under medium or low-load conditions. The basic problem which is to be solved by such prior art systems and by the present invention (considering the case of operating an induction motor at a single drive frequency) is to ensure that the supply voltage of the motor under full-load conditions will provide sufficient torque to balance the applied load, while also ensuring that an excessively high supply voltage (with correspondingly excessive power consumption) will not be applied when the motor load becomes very light, but at the same time ensuring that the supply voltage applied in that light-load condition will not be so low that stalling or instability of motor operation will result.
In addition, considering the case of variable-frequency drive of the induction motor, the appropriate supply voltage for any particular value of motor load will vary in accordance with the drive frequency, and the control system must therefore also modify the motor supply voltage in accordance with frequency.
One induction motor control method which has been proposed in U.S. Pat. No. 4,052,648 for achieving such objectives is based upon detecting the power factor phase angle (or the power factor itself, which is the cosine of the angle expressed as a value in the range 0% to 100% ) at which the motor is operating, and controlling the supply voltage of the motor such as to maintain the power factor at a certain preset value. If the motor is operating under a high load, with an appropriate value of supply voltage being applied, the power factor might for example be 80%. If now the load is increased, the amount of lag between the motor current and voltage will increase, so that the power factor will decrease. Conversely, a decrease in the motor load will result in an increase in the power factor. Thus, changes in the motor load can be detected by changes in the power factor, and the supply voltage can be controlled in responses to such detected changes in load, such as to hold the power factor at the preset value. However in practice with such a method, if the preset power factor is selected to be optimum for operation of the motor at full load, the voltage that is supplied when the motor is operating under light load will not be optimum for that value of load (i.e. optimum with regard to minimum power consumption consistent with prevention of stalling or instability). Similarly, when the motor is operated at an intermediate level of load, the supply voltage will not be appropriate for that load value.
Another proposal for an induction motor control system is described in U.S. Pat. No. 5,010,287. That system is designed for application to variable-frequency drive of an induction motor, to provide variable-speed operation. However considering operation with that system at any one particular drive frequency, the operation is similar to that described above, in that the power factor phase angle of the motor is controlled to be held at a preset value. Specifically, the actual power factor phase angle is detected and compared with a preset value of power factor phase angle (determined by pulses produced from a reference pulse generator), and the supply voltage is increased or decreased in accordance with the comparison result such as to bring the detected power factor phase angle to the preset value. Thus, the motor will only be operated at an optimum power factor when one specific load (referred to as the "standard load") is being imposed.
With that control system, respectively different values of preset power factor phase angle are selected in accordance with different values of drive frequency. However it can be understood from considering the case of operation at any specific value of drive frequency, that the same disadvantages arise as for the first-mentioned prior art U.S. patent, i.e. high efficiency and stability of operation cannot be ensured for operation over a wide range of varying motor loads, since the power factor that is established for the motor operation will only be optimum when the "standard load" is imposed.
The above point will be described referring to FIG. 1, in which the graph B represents the optimum values of power factor of an induction motor in the range from no-load to full-load (100%). It is assumed that the optimum power factor for full-load operation is 80%. With the prior art induction motor control systems described above, the power factor is controlled to be held at a value such as 80% (i.e. the broken-line characteristic A). However considering operation with a load that is 50% of full load, the optimum power factor might actually be 64%, and under the no-load condition the system will still attempt to maintain the power factor at 80%. Thus, such prior art systems which are based on power factor detection cannot provide optimum efficiency of operation over a wide range of values of imposed motor load.